This invention relates generally to electrostatographic reproduction machines, and more particularly to a fusing apparatus including a pneumatic member for increasing fusing nip width, and fusing dwell time.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the electrostatic latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roller or to an electrostatic latent image on the photoconductive member and form a toner powder image on the photoconductive member. The toner powder image is transferred from the photoconductive member to a copy substrate. The toner powder image is then heated to permanently fuse and affix it to the copy substrate.
In order to fix or fuse toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner images onto a supporting substrate has been to pass the substrate with unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated, i.e. a heated fuser roller. During operation of a fusing system of this type, the substrate to which the toner images are electrostatically adhered is moved through a nip formed between the roller members with the toner image contacting the heated fuser roller to thereby effect heating of the toner images within the nip. In a Nip Forming Fuser Roller (NFFR), the heated fuser roller is provided with a layer or layers that are deformable by a harder pressure roller when the two rollers are pressure engaged. The length of the nip determines the dwell time or time that toner particles remain in contact with the surface of the heated fuser roller.
Roller fusers work very well for fusing color and monochrome images at low speeds since the required process conditions such as temperature, pressure and dwell time can easily be achieved. When process speeds approach 100 pages per minute (ppm) roller fusing performance starts to falter. At such high speeds, dwell time must remain constant which necessitates an increase in nip width. Increasing nip width can be accomplished most readily by either increasing the fuser roller (FR) rubber thickness and/or the outside diameter of the fuser roller. Each of these solutions reach their limit at about 100 ppm. Specifically, the rubber thickness is limited by the maximum temperature the rubber can withstand and the thermal gradient across the elastomer layer. The fuser roller size becomes a critical issue for reasons of space, weight, cost, and stripping.
Thus conventional attempts to produce long fusing nips have tended to increase fuser roller diameter or require use of fusing belts. These approaches have their drawbacks which include increasing the overall size and hence the cost of the fusing apparatus. Additionally, current rubber pressure roller technology is also at its limit as far as increasing nip width by using softer materials.
According to the present invention, there is provided a compact long nip fusing apparatus including (a) a frame; (b) a rotatable first member for mounting to the frame; (c) a heat source for heating the rotatable first member; and (d) a rotatable and compressable pneumatic member forming a long fusing nip against the rotatable first member for contacting and fusing a fusible image.